Post-fire structural material behaviour of high- and ultra-high-strength steels – Experimental study and aids for assessment for further use and potential reuse after fires

A thorough understanding and realistic modelling of the essential post-fire material behaviour is crucial for a safe assessment of the residual structural performance of steel components and the reliable further usage of steel structures after fires. This paper presents a comprehensive experimental study of the constitutive material behaviour after heating-cooling cycles of two quenched and tempered high-strength steels (S690QL, S960QL) as well as two thermomechanically rolled steels (S960M, S1100M). The results are compared to both the behaviour of grade S355J2+N mild steels and a comprehensive database of existing post-fire tests. Except for high- and ultra-high-strength steels, which are exposed to very high temperatures above the A1-phase-transformation-temperature during fires, the assumption of reversibility of the mechanical properties for structural designs of steel structures has been confirmed. At high and very high steel temperatures, the microstructure changes and the strength properties after fire are significantly lower than those of virgin high-strength steel specimens. The study thus provides the basis for reliable structural designs and future normative guidance of the post-fire structural steel designs.